1. Field of the Invention
The present invention relates to a driving system for variably controlling of the number revolutions per time of an induction motor, and more particularly, to a driving system featuring an excellent responsiveness to a speed command.
2. Description of the Related Art
It is well known that each switching device of an inverter which supplies the current for driving each phase of an induction motor is turned ON or OFF in accordance with a sinusoidal wave PWM driving signal generated in response to a speed command, thereby performing variable control of the revolution speed of the motor. It is known that, in the sinusoidal wave PWM driving method, the responsiveness to a speed command is improved by carrying out vector control which permits instantaneous torque control of a motor by controlling the phases of driving signals. However, there is limitation in the switching performance of the switching devices of the inverter. Thus, it is difficult for the sinusoidal PWM driving method to properly control a motor's speed when the motor is in operation at such a high-speed as 40,000 revolutions or more per minute.
It is also known to variably control the revolution speed of an induction motor with ON-OFF control of switching devices of an inverter, using a square-wave signal. According to the square-wave drive control method, a motor's speed can be properly controlled even in a high-speed revolution range of the motor. However, in the square-wave drive control method, the vector control, which permits the control of an instantaneous torque of a motor, cannot be performed; therefore, slip control is employed so that a slip frequency can be controlled according to a motor revolution speed and a speed deviation thereof from a speed command to permit control of the average torque of the motor. Thus, the square-wave drive control method is disadvantageous in that it has poor responsiveness to the speed command.